This invention relates generally to the detection and processing of magnetically recorded digital data and particularly to a novel method and circuitry for accurately distinguishing the values of each read signal magnetically recorded in ternary 3-position modulation.
The 3-position modulation (3PM) system is described in detail in U.S. Pat. No. 4,323,931, issued April 6, 1982 to George V. Jacoby. Very briefly, the 3PM recording system is a self-clocking system that transforms a binary data word of, for example, three data bits into a bit stream that will be recorded in six positions or detents occupying the same track length of the three-bit binary word but with magnetic flux reversals no closer together than three of such detents, or one and one-half data bit lengths. The 3PM encoding technique permits a 50% increase in recorded data or bit densities over that available in NRZ or MFM code recording.
A further 33.3% increase in bit density over those resulting from 3PM encoding, and therefore still greater memory capabilities of a given magnetic memory, is achieved by the ternary 3PM encoding technique described in the copending application Ser. No. 260,248. Briefly, the ternary system employs three values (0, 1, and 2) into which the binary word is translated for recording on the magnetic medium. Thus, for example, a binary word comprising the two digits, 01, may be transformed into the ternary code 020, the digits 10 into ternary 100, and the digits 11 into ternary 200. As with 3PM encoding, ternary 3PM values are adjusted to assure that at least two zeros separate each recorded "1" or "2" so that the read signals representing these values may be readily distinguished. There ternary 3PM values are recorded on the magnetic medium as typical square waveform signals with a ternary "1" represented as a single flux change, a ternary "2" as a square wave pulse of two flux changes per detent, and a ternary "0" represented by no flux changes. At the very high recording densities, the output waveforms from the read head appear to be almost indistinguishable and incapable of rendering useful data signals, until they are reformed by suitable read equalizer circuits.
This invention is for a method and circuitry for the detection of equalized ternary 3PM analog read signals and the accurate identification of the values represented therein.